Generally, conventional metal halide arc discharge lamps are of the so-called "double-ended" design, as disclosed by Koury in U.S. Pat. No. 3,407,327. Therein, a pair of electrodes are sealed into opposite ends of a fused silica arc tube, an appropriate fill gas is disposed within the arc tube and the arc tube is located within an outer envelope having an attached base member formed for acceptance by a socket. Moreover, commercially available units range from 175 to 1500 watts with an arc gap between the electrodes in the range of 26 to 91 mm and a gap tolerance as great as .+-.1.0 mm.
An improvement over the above-described relatively high voltage metal halide arc discharge lamp is set forth in U.S. Pat. No. 4,161,672 issued to Cap et al. Herein, a lamp is disclosed having a power input of about 250-watts and an arc loading of about 60 to 150 watts/cm. However, this structure is again of a "double-ended" variety wherein an electrode is sealed into each end of a longitudinal-extending arc tube. Unfortunately, "double-ended" structures are relatively expensive to manufacture, are not especially suited to an outer envelope with a connecting base at one end due to the double-ended configuration of the arc lamp and tend to exhibit relatively large tolerances in so far as gap spacing and positioning of the electrodes is concerned.
As even greater improvement in the provision of a light source is set forth in the single-ended metal halide arc discharge lamps of U.S. Pat. Nos. 4,302,699; 4,308,483; 4,320,322; 4,321,501 and 4,321,504. All of the above-mentioned patents disclose structures and/or fill gas variations suitable to particular applications. However, each leaves something to be desired insofar as the cost of electrode and arc chamber alignment and tolerances are concerned.